1. Field of the Invention (Technical Field)
The present invention relates to power source electrolytes, both nonaqueous liquid electrolytes and solid polymer electrolytes (SPEs).
2. Background Art
Conventional electrolyte salts commonly used in nonaqueous electrolytes have the high conductivities required for designing high power, rechargeable batteries for electric vehicle applications. Unfortunately, metallic lithium reacts with the nonaqueous electrolytes and electrolyte salts, and these unwanted reactions cause premature cell failure. During charge/discharge cycling, electrolyte decomposition takes place and the decomposition products appear to react with the polymer separator. Deterioration of the separator can also cause cell failure due to internal shorts.
The most advanced rechargeable lithium batteries employ a negative electrode composed of either metallic lithium or a carbon intercalation compound and a positive electrode composed of an inorganic intercalation compound, and a liquid organic electrolyte. Energy densities greater than 100 Wh/kg have been achieved in small prototype cells with Li.sub.x CoO.sub.2, Li.sub.x NiO.sub.2, Li.sub.x Mn.sub.2 O.sub.4, and Li.sub.x TiS.sub.2 positive electrodes. The drawbacks with these systems are safety, high cost, poor utilization of positive electrodes, poor lithium plating efficiency when metallic lithium is used, and hence poor cycle life and electrolyte decomposition during cell cycling. The power densities required for electric vehicle batteries may not be achievable with organic liquid electrolytes if the electrolytic decomposition results in electrode passivation.
Ionically conductive solid polymer electrolytes (SPEs) have been proposed to alleviate these problems. Significant research activities to produce highly conductive solid polymer electrolytes are being conducted throughout the world. The solid polymer electrolytes are electrical insulators, and therefore the use of a separator film is not needed. The common objective of the various investigations is to develop an SPE system that can be used in practical applications. Presently available liquid and SPEs are made with electrolyte salts dissolved in organic solvents, and, in the case of SPEs, immobilized in a polymer such as poly(ethylene oxide) (PEO) or poly(acrylonitrile) (PAN) polymers. The commonly used salts are LiAsF.sub.6, LiPF.sub.6, LiClO.sub.4, LiCF.sub.3 SO.sub.3, and the commonly used organic solvents are: ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, .gamma.-butyrolactone, dimethylformamide, etc. The electrolytes are unstable at lithium potentials; therefore, they undergo degradation on prolonged cycling, thus causing premature cell failure. In addition to the instability, because of the facile movement of both cations and anions of the electrolyte, unacceptable accumulation of these salts takes place at the electrode interphase on prolonged cycling or storage.
The present invention is of novel electrolyte salts for both organic liquid and solid polymer electrolytes that exhibit many beneficial properties, such as excellent thermal stability, good ionic conductivity, a wide electrochemical window, and ability to function with and without the use of an organic solvent, as described more fully below. The preferred electrolyte has a very large anion moiety, but the equivalent weight per lithium cation is smaller than hexafluoroarsenate or many other known electrolyte salts presently being used.